A wheelchair enables a person otherwise unable to move about to move from place to place with a degree of freedom. Motorized wheelchairs enable a user to move about with relative ease by manipulating a few wheelchair controls. Wheelchair controls are typically hand operated devices but there are wheelchair controls adapted for special needs. A special need may exist where the wheelchair user does not have use of the hands to operate the controls.
These special needs have been addressed in part by providing wheelchair control systems that rely on leg operated, chin operated and head operated controls. Controls that utilize movement of the head to activate wheelchair controls exist which use reflecting mirrors attached to the head and movable with the head while others use ultrasonic techniques to bounce sound waves from the head and thus determine its position for controlling the wheelchair. Wheelchair control systems of these types are described in U.S. Pat. Nos. 3,374,845; 3,965,402; 3,993,154; 4,281,734; 4,093,037; and 4,679,644.
U.S. Pat. No. 3,761,736 describes a proximity switch operating on a capacitance principle to detect changes in capacitance occurring when an object comes in close proximity to the switch. Capacitance principles are also employed in the apparatus of U.S. Pat. No. 3,993,154 wherein a low power signal is carried by a conductor to a thin conductive metal foil located in a back rest portion of a motorized wheelchair. Conductive metal field pickup or skin contact elements are supported at a location on the wheelchair where they may be momentarily contacted by any exposed skin portion of the wheelchair user. The conductive field pickup elements are separated from each other so that they may be individually and selectively contacted. The foil acts as a radiating source by virtue of being connected to an oscillator to thereby radiate the energy field into the body of the wheelchair user.
A drawback of sensing circuits used for wheelchair controls is that contact with the body is required. Such contact may take the form of a helmet or mirrors attached to the head or may take the form of physical body contact with part of a wheelchair. It will be appreciated that it would be highly desirable to provide a sensing circuit for a wheelchair control that does not involve physical contact with the user's body and does not hinder the user's field of vision or mobility.
Another sensing circuit utilizing capacitive elements uses a pair of capacitors in conjunction with a diode bridge. This circuit uses a pair of capacitors to produce a speed signal and another pair to produce a turn signal. In controlling wheelchair speed, one plate of a capacitor is attached to a movable portion of the wheelchair or a movable plate attached to the wheelchair. Understandably, with both the speed sensor and the turn sensor requiring two capacitors each, the circuitry tends to be fairly complicated and bulky. In addition, movement of at least two body parts is required with one part used to vary speed and the other used to control turn direction. Also, the diode bridge-capacitor type detector circuit detects spontaneously anything that is fed to it and is therefore highly subjective to external field effects, such as radio frequency waves. Accordingly, it will be appreciated that it would be highly desirable to have a simple sensing circuit for a wheelchair control providing a simple, nonobtrusive interface with the user that is free of interference from ever present radio frequency waves.